I. Field of the Invention
This invention relates generally to controls for motors. In particular, this invention relates to controls for brushless DC motors.
II. Description of the Prior Art
Brushless DC motors of the type of interest herein are constructed to have a plurality of field coils arranged in a stator and a rotor rotatable relative thereto having a plurality of permanent magnetic pole pieces arranged therein. Such motors permit control of velocity over a wide range as is inherent with DC motors and eliminate the inherent disadvantages of brush wear and attendant loss of motor performance. "Commutation" of current in motors of this type occurs in the field coils and is accomplished by electronically controlled switches. The commutation of current in the field coils necessitates the detection of rotor position to identify the coils to which energy is to be applied to produce the desired torque.
It is known to control brushless DC motors with primarily analogue devices providing the desired compensation of the motor and control responses to effect a desired motor performance. In such known controls, commutation may be effected at the switching stage by direct application of the output of the rotor position transducer. To this end it is common to use a Hall effect transducer mounted to the motor rotor to determine rotor relative position and control the field coil current switches. In such control systems measurement of motor angular velocity, required for control of torque, is typically accomplished by a conventional tachogenerator. When it is desired to measure position of a machine element driven by the motor, further position transducers are used.
The known analogue controls for brushless DC motors suffer the disadvantage of requiring manual adjustment at the control device of variable parameters and lack programmable adaptability for large variations in motor sizes, loads and speeds. Further, the use of the various dedicated transducers described above in conjunction with such known control systems increases the cost of such systems, while adding to the complexity thereof, thereby reducing overall reliability.